1. Field of the Invention
The present invention relates to a method of manufacturing a radially oriented magnet, and more particularly to a method of manufacturing a radially oriented magnet for use in a small-sized motor or the like.
2. Description of the Prior Art
A radially oriented magnet is directed to) a ring-shaped magnet which has been manufactured by sintering or curing magnetic powders after they have been radially oriented (radial orientation). FIG. 3 shows a conventional method of manufacturing a radially oriented magnet. A mold structure as shown is provided with a magnetic yoke 11 formed of an electromagnet. The magnetic flux by the electromagnet 11 is induced in a magnetic circuit 12 as broken lines. Magnetic powder which is filled in the hole between a pair of upper and lower punches 13, 13a of a non-magnetic metallic yoke and is radially oriented by the magnetic circuit as shown in the figure, and then compressed in a ring shape by the upper punch 13.
Thus, the magnetic flux is induced in the magnetic circuit with the magnetic yoke and the magnetic powder oriented in radial directions in the magnetic circuit, and then the magnetic powder is press-formed in the magnetic field, thereby producing the conventional radial oriented magnet. In many cases, a steady magnetic field generated by an electromagnet is utilized, however, there is also a method of utilizing a pulse magnetic field.
Particularly, in the case where the yoke formed of a magnetic material inserted inside of the ring to form a magnetic circuit, the yoke material is liable to be saturated more as the cross sectional area of the yoke is reduced. Therefore, a magnetic field for the radial orientation of the magnetic powder is insufficient enough (for example, refer to Japanese Patent Unexamined Publication No.. Hei 2-281721, Japanese Patent Unexamined Publication No.. Hei 2-18905, Japanese Patent Unexamined Publication No.. Sho 63-310356, and the like), and therefore there is a drawback that a radial oriented magnet having a desired characteristic cannot be manufactured.
Further, in the case of using a pulse magnetic field, because a magnetic material and a non-magnetic material such as the punches, the die and the like are arranged at such positions that the magnetic flux is changed in a pulse manner, the magnetic field cannot be satisfactorily inserted into the magnetic material due to a skin effect of eddy current, and the magnetic field does not have a radial orientation, resulting in a drawback that the degree of orientation is extremely lowered.
The present invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a method of manufacturing a radially oriented magnet which is downsized and has a high degree of orientation.